[Show abstract][Hide abstract] ABSTRACT: The acoustic properties of two clinical (Definity, Lantheus Medical Imaging, North Billerica, MA, USA; SonoVue, Bracco S.P.A., Milan, Italy) and one pre-clinical (MicroMarker, untargeted, Bracco, Geneva, Switzerland; VisualSonics, Toronto, ON, Canada) ultrasound contrast agent were characterized using a broadband substitution technique over the ultrasound frequency range 12-43 MHz at 20 ± 1°C. At the same number concentration, the acoustic attenuation and contrast-to-tissue ratio of the three native ultrasound contrast agents are comparable at frequencies below 30 MHz, though their size distributions and encapsulated gases and shells differ. At frequencies above 30 MHz, native MicroMarker has higher attenuation values and contrast-to-tissue ratios than native Definity and SonoVue. Decantation was found to be an effective method to alter the size distribution and concentration of native clinical microbubble populations, enabling further contrast enhancement for specific pre-clinical applications.
Ultrasound in medicine & biology 12/2013; · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sub-capillary sized microbubbles offer improved techniques for diagnosis and therapy of vascular related disease using ultrasound. Their physical interaction with ultrasound remains an active research field that aims to optimize techniques. The aim of this study is to investigate whether controlled microbubble disruption upon exposure to consecutive ultrasound exposures can be achieved. Single lipid-shelled microbubble scattered echoes have been measured in response to two consecutive imaging pulses, using a calibrated micro-acoustic system. The nonlinear evolution of microbubble echoes provides an exact signature above and below primary and secondary resonance, which has been identified using theoretical results based on the Mooney-Rivlin strain softening shell model. Decaying microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures. The characteristics of the microbubble motion causes varying amounts of shell material to be lost during microbubble decay. Incident ultrasound field parameters can thus accurately manipulate the regulated shedding of shell material, which has applications for both imaging applications and localized drug delivery strategies.
Physics in Medicine and Biology 01/2013; 58(3):589-599. · 2.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In diagnostic medicine, microbubbles are used as contrast agents to image blood flow and perfusion in large and small vessels. The small vessels (the capillaries) have diameters from a few hundred micrometers down to less than 10 μ m. The effect of such microvessels surrounding the oscillating microbubbles is currently unknown, and is important for increased sensitivity in contrast diagnostics and manipulation of microbubbles for localized drug release. Here, oscillations of microbubbles in tubes with inner diameters of 25 μm and 160 ¿m are investigated using an ultra-high-speed camera at frame rates of ~12 million frames/s. A reduction of up to 50% in the amplitude of oscillation was observed for microbubbles in the smaller 25-μm tube, compared with those in a 160-μm tube. In the 25-μm tube, at 50 kPa, a 48% increase of microbubbles that did not oscillate above the noise level of the system was observed, indicating increased oscillation damping. No difference was observed between the resonance frequency curves calculated for microbubbles in 25-μm and 160-μm tubes. Although previous investigators have shown the effect of microvessels on microbubble oscillation at high ultrasound pressures, the present study provides the first optical images of low-amplitude microbubble oscillations in small tubes.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 01/2013; 60(1):105-14. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Controlled microbubble stability upon exposure to consecutive ultrasound exposures is important for increased sensitivity in contrast enhanced ultrasound diagnostics and manipulation for localised drug release. An ultra high-speed camera operating at 13 × 106 frames per second is used to show that a physical instability in the encapsulating lipid shell can be promoted by ultrasound, causing loss of shell material that depends on the characteristics of the microbubble motion. This leads to well characterized disruption, and microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures.
[Show abstract][Hide abstract] ABSTRACT: This study characterized the acoustic properties of an International Electromechanical Commission (IEC) agar-based tissue mimicking material (TMM) at ultrasound frequencies in the range 10-47 MHz. A broadband reflection substitution technique was employed using two independent systems at 21°C ± 1°C. Using a commercially available preclinical ultrasound scanner and a scanning acoustic macroscope, the measured speeds of sound were 1547.4 ± 1.4 m∙s(-1) and 1548.0 ± 6.1 m∙s(-1), respectively, and were approximately constant over the frequency range. The measured attenuation (dB∙cm(-1)) was found to vary with frequency f (MHz) as 0.40f + 0.0076f(2). Using this polynomial equation and extrapolating to lower frequencies give values comparable to those published at lower frequencies and can estimate the attenuation of this TMM in the frequency range up to 47 MHz. This characterisation enhances understanding in the use of this TMM as a tissue equivalent material for high frequency ultrasound applications.
Ultrasound in medicine & biology 04/2012; 38(7):1262-70. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study uses atomic force microscopy (AFM) force-deformation (F-Δ) curves to investigate for the first time the Young's modulus of a phospholipid microbubble (MB) ultrasound contrast agent. The stiffness of the MBs was calculated from the gradient of the F-Δ curves, and the Young's modulus of the MB shell was calculated by employing two different mechanical models based on the Reissner and elastic membrane theories. We found that the relatively soft phospholipid-based MBs behave inherently differently to stiffer, polymer-based MBs [Glynos, E.; Koutsos, V.; McDicken, W. N.; Moran, C. M.; Pye, S. D.; Ross, J. A.; Sboros, V. Langmuir2009, 25 (13), 7514-7522] and that elastic membrane theory is the most appropriate of the models tested for evaluating the Young's modulus of the phospholipid shell, agreeing with values available for living cell membranes, supported lipid bilayers, and synthetic phospholipid vesicles. Furthermore, we show that AFM F-Δ curves in combination with a suitable mechanical model can assess the shell properties of phospholipid MBs. The "effective" Young's modulus of the whole bubble was also calculated by analysis using Hertz theory. This analysis yielded values which are in agreement with results from studies which used Hertz theory to analyze similar systems such as cells.
[Show abstract][Hide abstract] ABSTRACT: Nonlinear microbubble oscillations can be exploited by signal processing to provide contrast enhancement in ultrasound imaging. High-speed optical investigations have established that microbubbles resonate in-vitro, but resonance has not yet been established in-vivo. Here we aim to establish the acoustic signature of resonance, which will help identify its occurrence in-vivo and further optimize microbubble signal processing. Resonant microbubbles provide a transient acoustic amplitude response, which is a signature unique to resonance. Large numbers of acoustic signals from single lipid-shelled Definity® microbubbles have been measured using a calibrated micro-acoustic system and a unique transient characteristic of resonance has been identified in the onset of scatter. We present theoretical results based on the Mooney-Rivlin strain softening shell model to show that for realistic shell parameters pulse durations longer than those routinely used in clinical imaging pulses are required to reach a steady state microbubble resonance, indicating the majority of contrast enhancement arises from off-primary resonance microbubble scatter. The specificity of this response may generate higher sensitivity signal processing algorithms, and this should be investigated in the future in realistic vessel sizes to address in vivo requirements.
[Show abstract][Hide abstract] ABSTRACT: Microbubbles are used to improve ultrasound imaging of the vascular bed. Optical microscopy has shown microbubbles in different size tubes which have different responses to ultrasound. The acoustic scatter associated with such differences has not been previously measured. Echoes from two types of microbubbles, in narrow tubes, were collected at incident ultrasound parameters relevant to diagnostic imaging. Microbubbles were found to have increased second harmonic signatures in 50 μm diameter tubes compared to 200 μm. There was decreased survival of lipid microbubbles in the smaller tube. Understanding scatter mechanisms in narrow tubes is useful for signal processing optimisation for imaging applications.
[Show abstract][Hide abstract] ABSTRACT: High frequency ultrasound (>;15 MHz) is used in the fields of pre-clinical, intravascular, ophthalmology and superficial tissue imaging. Tissue-equivalent phantoms have previously been developed to enable relevant quality assurance measurements for frequencies lower than 15MHz but to date limited data is available on the acoustic properties of tissue equivalent material at frequencies higher than 15MHz. In this study, the acoustic attenuation of an International Electromechanical Commission (IEC) agar based tissue mimicking material (TMM) was measured over the frequency range of 12 - 47 MHz at 21 °C ± 1 °C using a broadband substitution technique. By applying a polynomial curve fitting function to the measured data the attenuation (dB·cm-1) was found to vary with frequency f(MHz) in the form of 0.39 f + 0.0069 f2. This result is comparable to the published result at lower frequencies.
[Show abstract][Hide abstract] ABSTRACT: Ultrasound contrast agents have been the subject of microvascular imaging research. The sheep corpus luteum (CL) is a microvascular tissue that provides a natural angiogenic and antiangiogenic process, which changes during the luteal phase of the estrous cycle of the ewe. It can also be controlled and monitored endocrinologically, providing a very attractive in vivo model for the study and development of microvascular measurement. The perfusion of the fully developed CL between days 8 and 12 of the estrous cycle was studied in six ewes. A Philips iU22 ultrasound scanner (Bothell, WA, USA) with the linear array probe L9-3 was used to capture contrast-enhanced images after an intravenous bolus injection of 2.4 mL SonoVue (Bracco S.P.A., Milan, Italy). Time-intensity curves of a region of interest inside the CL were formed from linearized image data. A lagged-normal model to simulate the compartmental kinetics of the microvascular flow was used to fit the data, and the wash-in time was measured. Good contrast enhancement was observed in the CLs of all animals and the wash-in time averaged at 5.5 s with 9% uncertainty. The regression coefficient was highly significant for all fits. These data correlated with stained endothelial area in the histology performed postmortem. Two ewes were injected with prostaglandin F2alpha to induce CL regression, which resulted in an increase of wash-in time after a few hours. The CL of the ewe is thus proposed as an ideal model for the study and development of microvascular measurements using contrast ultrasound. Our initial results demonstrate a highly reproducible model for the study of the microvascular hemodynamics in a range of tissues and organs.
Ultrasound in medicine & biology 01/2011; 37(1):59-68. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The understanding and exploitation of acoustic echo signals from nonlinear ultrasound scatterers is an active research area that aims to improve the sensitivity and specificity of diagnostic imaging. Discriminating between acoustic echoes from linear scatterers, such as tissue, and nonlinear scatterers, such as contrast microbubbles, based on their frequency content is also an important topic in ultrasound contrast imaging. In order to achieve these objectives, a fundamental preliminary stage is to extract information about the reflected signals in the frequency domain with high accuracy: this is essentially a feature extraction and estimation problem. In this paper, a parametric Bayesian spectral estimation method is utilised for the analysis of the backscattered echo signals from microbubbles. In contrast to existing nonparametric discrete-Fourier-transform- (DFT-) based spectral estimation techniques used in the ultrasonic literature, this method is able to estimate the number of spectral components as well as their amplitudes and frequencies. The Bayesian spectral analysis technique has improved frequency resolution compared with the DFT for shortmultiple-component signals at low signal-to-noise ratios. The performance of the method is demonstrated with simulated signals, as well as analysing experimentally measured echo signals from nonlinear microbubble scatterers.
[Show abstract][Hide abstract] ABSTRACT: Single microbubble acoustic acquisitions provide information on the behaviour of microbubble populations by enabling the generation of large amounts of data. Acoustic signals from single polylactide-shelled and albumin coated biSphere™ microbubbles have been acquired. The responses observed from a range of incident frequencies and acoustic pressures varied in duration. Partial echoes shorter than the incident pulse duration have been observed for low frequency pulses of sufficient amplitude, suggesting release of gas from bubbles. The results presented suggest that the mechanism of scatter from hard shelled agents may be shell disruption and gas release, or partly from gas leaking from defected shell sites, which has previously not been observed optically. These results can provide the basis for improved imaging through optimization of incident pulse parameters, with potential benefits to both diagnostic and therapeutic techniques.
Ultrasound in medicine & biology 11/2010; 36(11):1884-92. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microbubble science is expanding beyond ultrasound imaging applications to biological targeting and drug/gene delivery. The characteristics of molecular targeting should be tested by a measurement system that can assess targeting efficacy and strength. Atomic force microscopy (AFM) is capable of piconewton force resolution, and is reported to measure the strength of single hydrogen bonds. An in-house targeted microbubble modified using the biotin-avidin chemistry and the CD31 antibody was used to probe cultures of Sk-Hep1 hepatic endothelial cells. We report that the targeted microbubbles provide a single distribution of adhesion forces with a median of 93pN. This interaction is assigned to the CD31 antibody-antigen unbinding event. Information on the distances between the interaction forces was obtained and could be important for future microbubble fabrication. In conclusion, the capability of single microbubbles to target cell lines was shown to be feasible with AFM.
[Show abstract][Hide abstract] ABSTRACT: Indicator dilution methods have a long history in the quantification of both macro- and microvascular blood flow in many clinical applications. Various models have been employed in the past to isolate the primary pass of an indicator after an intravenous bolus injection. The use of indicator dilution techniques allows for the estimation of hemodynamic parameters of a tumor or organ and thus may lead to useful diagnostic and therapy monitoring information. In this paper, we review and discuss the properties of the lognormal function, the gamma variate function, the diffusion with drift models, and the lagged normal function, which have been used to model indicator dilution curves in different fields of medicine. We fit these models to contrast-enhanced ultrasound time-intensity curves from liver metastases and the ovine corpora lutea. We evaluate the models' performance on the image data and compare their predictions for hemodynamic-related parameters such as the area under the curve, the mean transit time, the full-width at half-maximum, the time to the peak intensity, and wash-in time. The models that best fit the experimental data are the lognormal function and the diffusion with drift.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 07/2010; · 1.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The study of single microbubble (MB) acoustics has relevance to signal and image processing of ultrasound (US) contrast imaging. The aim of this work is to study the effect of consecutive pulses on MB response. RF backscatter data from 223 single lipid shelled Definity® MBs were collected using a flow phantom and a modified Philips Sonos 5500 scanner. The results from 2 consecutive imaging pulses (1.6MHz, 550kPa, 6 cycles, 1kHz PRF) are presented and compared to similar results from a theoretical model of Definity (Keller-Miksis equation, Mooney-Rivlin shell model). Bubble survival in the 2nd pulse response was observed for 64% of scatterers. Surviving signals were observed to decay in amplitude and increase in relative harmonic content with each incident pulse, with the rate of decay being dependant on the first pulse response. Signals from MBs at resonance and above, are more resilient to the incident pulse compared to off-resonant MBs. Resonant MBs have been observed to migrate from resonant scatter in the first pulse response to off-resonant scatter in subsequent pulses, suggesting a change in size which takes them across the resonance peak. Theoretical results confirm that realistic changes in size can account for such differences, and will lead to an increase in the relative harmonic components as measured here. In light of these findings, the larger decay of off-resonant scatter compared to resonant scatter may offer increased performance of pulse sequences such as pulse amplitude modulation, and the order of incident pulses in the sequence then becomes crucial. Mechanisms including acoustically- driven diffusion and lipid shedding could be incorporated in future theoretical models to account for these effects.
[Show abstract][Hide abstract] ABSTRACT: Imaging microvascular flow is of diagnostic value for a wide range of diseases including cancer, inflammation, and cardiovascular disease. The introduction of microbubbles as ultrasound contrast agents offers significant signal enhancement to the otherwise weakly scattered signal from blood in the circulation. Microbubbles provide maximum impedance mismatch, but are not linear scatterers. Their complex response to ultrasound has generated research on both their behaviour and their scattered-signal processing. Nearly 20 years ago signal processing started with simple spectral filtering of harmonics showing contrast-enhanced images. More recent pulse encoding techniques have achieved good cancellation of tissue echoes. The good quality contrast-only images enabled ultrasound contrast-imaging applications to be established in microvascular measurements in the liver and the myocardium. The field promises to advance the quantification of microvascular flow kinetics.
Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 01/2010; 224(2):273-90. · 1.42 Impact Factor